A number of metallurgical processes involve the treatment of aqueous slurries containing solid, metal-containing particles. In one such metallurgical process, an aqueous slurry of a mineral or metal-containing compound is subjected to leaching or oxidation at elevated temperatures and pressures in an autoclave. In order to maximize the efficiency of the leaching or oxidation process, it is preferred to preheat the slurry before feeding it into the autoclave. For this purpose, a heater vessel is provided upstream of the autoclave. The slurry is heated by contact with a heating medium inside the heater vessel. The heating medium typically comprises steam generated by an autoclave flashing process.
It is preferred that the temperature of the slurry be heated to within a narrow temperature range inside the heater vessel. The temperature of the discharge slurry is typically maintained by controlling the flash steam pressure in the apparatus (by venting). However, precise control of the slurry temperature is difficult to achieve in this manner as there are a number of other process variables such as autoclave throughput, heating medium composition, feed slurry specific heat capacity and boiling point elevation, which affect the quantity and quality of heating medium condensed by the process, and control of the vent gas flow is not always practical.
Accordingly, there is a need for improved apparatus and methods for heating slurries in a heater vessel.